6L6 to EL34 Mod for Fender Bassman Heads
6L6 power tubes can be a little too clean for the tastes of many. One option is to use 5881's. These are direct replacements for the 6L6 which require nothing more than a
rebias after installation. They give a little less headroom, break up a little sooner, and add a little more grind to the amp.
Another option is to swap your 6L6's with EL34's. EL34's have a darker tone, the break up a lot earlier than the 6L6, and their overdriven sound is radically different
than a 6L6. If you're looking for a big change in tone, then this may be just what you're looking for. But EL34's ARE NOT direct replacements for the 6L6. You'll need to
make a slight modification to your amp so it can use them.
1. Replace the 470 ohm resistor that's between pins 4 and 6 of each output tube socket with a 1000 ohm, 2 watt resistor.
2. Disconnect pin 1 only of the 1500ohm resistor that's between pins 1 and 5 of each output tube socket.
3. Disconnect the wires on pin 1 of the output tube socket.
4. Reconnect the wires that were on pin 1, to the end of the 1500-ohm resisors that are now standing free on each output tube socket.
5. Connect pin 1 to pin 8 of each output tube socket with a solid wire.
6. Adjust the bias of the amp to suit the new tubes. See here for instructions on how to do it.
After this mod you can use either EL34's or 6L6's just by adjusting the bias. I have done this on my Silverface Bassman heads and on my Bassman 100's. Do it
I use this method to bias my amps. I also use the 70% of maximum dissipation rule I learned from Randy Aikens site. Below is an exerpt, read the whole thing here. In fact,
go to Randy's site and read all the tech stuff there. You'll be a lot smarter when yer done. - Regis
From Randy Aikens site:
A general rule of thumb is that class AB amplifiers are usually operated at no more than 70% of the maximum plate dissipation of the tube (to account for the higher
dissipation that occurs under signal conditions), while true class A amplifiers generally run right at the maximum plate dissipation (the dissipation at full power is
lower than the dissipation at idle in a true class A amplifier).
For example, the aforementioned EL34 tube has a plate dissipation of 25W, so at 400V class AB operation, it should be biased no higher than (0.7 * 25/400) = 44mA. At 500V
class AB operation, it should be biased no higher than (0.7 * 25/500) = 35mA.
This doesn't mean you should automatically bias all tubes to 70% of max dissipation!
They can be biased at any lower current if desired, and many people prefer a point of around 50% to 60% of the max plate dissipation, which contributes to longer tube
life.
In true class A operation at 250V, it should be biased no higher than (25W/250V) = 100mA. Note that a class A amplifier does not necessarily have to be run at the
maximum ratings. You could design a true class A amplifier at lower plate voltages and higher currents, but there is a limit to how high the plate voltage can be without
exceeding dissipation ratings, or having to go to class AB. There is also a limit at how high the plate or cathode current can be for a particular tube. A class B
amplifier should be biased right at cutoff, or perhaps a few mA standing current, to minimize crossover distortion. Class B amplifiers usually have extremely high plate
voltages in order to maximize the output power, so they must be biased right at cutoff to prevent over-dissipation at full power. If in doubt about the actual operating
conditions of the circuit, call the manufacturer or refer servicing to a qualified amp technician
